1. Field of the Invention present invention relates to servo-writers for magnetic disk drive memories.
2. Description of Related Art
Magnetic disk drive memories are well-known devices for storing large quantities of digital data. An example of a typical disk drive is indicated generally at 10 in FIG. 1. As the name suggests, a disk drive has one or more disks 12 coated with a magnetizable medium such as ferrous oxide on which the data may be magnetically stored. The disks, often referred to as platters, may be vertically stacked and caused to rotate about a central spindle 14. Data may be stored or "written" onto each surface of the media platters by read-write data heads 16 which are placed in close physical proximity to the surface of a platter. By passing a current through a read-write head, a small portion of the surface of the associated platter may be magnetized to form a magnetic domain and store a digital 1 or 0 onto the platter.
Typically, the read-write heads 16 are rigidly coupled to each other in a vertical stack, and may be moved together as an assembly 20 in a radial motion by a positioning mechanism 22 either towards or away from the spindle 14 of the platter stack. At each position of a head of the head stack, a unique circular track is defined in the surface of the associated platter as the platter rotates below the head. To read data written on the circular data tracks, currents induced in the read-write data heads by the magnetic domains on the media platter are sensed and amplified for transmission to other parts the computer system.
As data storage densities have increased, the spacing between data tracks has decreased. Many disk drives now have in excess of 2,000 tracks per inch of radial head movement. At such densities, special positioning mechanisms referred to as "servos", have been used to ensure proper positioning of the data heads over the data tracks of the disk drive. The servo mechanism of FIG. 1 includes a servo head 24 which reads position information from a special pattern of servo tracks written onto a surface 26 of one of the platters.
One type of servo system is referred to as the "dedicated servo" and employs a servo pattern written to the entirety of the platter surface 26. The position information read by the servo head is the sole source of positioning information for the head assembly.
Where high densities of data are required, the servo pattern is typically written by a special device referred to as a "servo pattern writer" or simply a "servowriter". To position the servo head for writing the servo pattern, a servowriter typically includes a precision lead screw mechanism which is usually controlled by an extremely accurate position control system such as a laser interferometer or an extremely high resolution optical encoder. The servo pattern to be written is typically generated by high speed, precisely clocked logic circuitry. Each disk drive manufacturer often has its own unique servo pattern which may even vary from model to model of the same manufacturer.
FIG. 2 illustrates a servo pattern for a model 6650 disk drive, manufactured by Priam Corporation. The servo pattern when read, allows the servo system to generate an error position signal relative to the current track being read. Often, the patterns of positioning pulses of the servo pattern are alternated from track to track to define "odd" and "even" servo tracks. The equilibrium position of the servo head is usually halfway between adjacent odd and even servo tracks. Thus, the signal read back from the servo head will be a composite of these adjacent even and odd tracks.
The servo system compares the amplitudes of the pulses of the read back signal at odd and even track positioning pulse time windows to generate a position error signal. When the servo head is centered precisely between the odd and even tracks, the odd and even position read back signals will be equal in amplitude, generating no error signal. However, if the servo head is off center between the even and odd tracks, an error signal is generated which is used to reposition the servo head appropriately. Since the data heads are coupled to the servo head, the data heads will follow the movement of the servo head. In this manner, the circular tracks of the servo pattern defines the data tracks on the other surfaces of the platter stack.
The servo pattern also can provide a timing reference. Typically, special timing reference pulses are included in the pattern in a manner in which the pulses can be detected, allowing a phase-locked loop to lock to the timing reference pulses during operation.
Another function of the servo pattern is to provide special position information to the servo system. For example, data may be encoded into the pattern to provide index and sector mark signals, indicating the angular position of the platter stack. Similarly, servo tracks outside of the range of data tracks may be encoded to form guard band zones.
Two primary types of servowriters are custom servowriters and configurable or universal servowriters. A custom servowriter is generally capable of writing a servo pattern for only one particular model of disk drives. Consequently, these servowriters typically generate only one type of servo pattern at a fixed frequency and are generally capable of positioning the heads at a single fixed track density.
Configurable or universal servowriters, on the other hand, are designed to be able to write a variety of servo patterns at different frequencies and track densities. The configurability or programmablity of a servowriter is highly desirable for several reasons. For example, the development of a particular disk drive must by accompanied by or perhaps even preceded by the availability of a servowriter which is capable of writing the servo pattern for that disk drive. Once a servowriter has been designed and constructed to write the selected servo pattern for a particular disk drive, a dedicated servowriter inhibits any modifications to the servo pattern. Furthermore, servowriters are relatively expensive devices, the cost of which can be ammortized more effectively if the servowriter can be adapted to other, or future disk drive designs. Still further, a servowriter which may be used for many different disk drive types is highly desirable for use in disk drive service and repair.
However, previous configurable servowriters have tended to be much more expensive than custom servowriters. For example, U.S. Pat. No. 4,584,619 to Nay, et al. describes a programmable servo pattern generator for a configurable servowriter which employs a pattern programming hierarchy. Different levels of pattern repetition are included in the hierarchy such as "cell", "frame", "sector", and "track type". The contents, length and repetition count of each level are programmable to an extent. A phase-locked loop of very high frequency (&gt;80 megahertz) enables the user to write the pattern transitions at any arbitrary interval subject to a small quantization error. The clock frequency is programmable. A wide selection of sensor, driver and power options, with elaborate programmable controls, may also be provided as an interface to the disk drive mechanism. One problem with this approach is that in addition to being quite expensive, the resultant servowriter is quite complex and bulky.